1. Field of the Invention
The present invention relates to a group III nitride compound semiconductor device and a producing method therefor. Particularly, it relates to improvement in a buffer layer-forming method used when a group III nitride compound semiconductor device is produced by a metal organic chemical vapor deposition method (MOCVD method).
The present application is based on Japanese Patent Applications No. Hei. 11-222882 and 11-315193, which are incorporated herein by reference.
2. Description of the Related Art
Heretofore known is a group III nitride compound semiconductor device having a structure in which group III nitride compound semiconductors are laminated on a sapphire substrate through a buffer layer of AlXGa1−XN (0≦X≦1).
For example, Unexamined Japanese Patent Publication No. Sho. 62-119196 has described a buffer layer-forming method in which a buffer layer of AlXGa1−XN (0≦X≦1) is grown at a growth temperature of from 950 to 1150° C. on a sapphire substrate heated to about 1000° C. by the MOCVD method.
On the other hand, by the investigation thereafter, it has been found that the crystallinity of a GaN compound semiconductor layer grown on a buffer layer is improved when the buffer layer is grown at a lower temperature of about 400° C. on a sapphire substrate heated to about 1000° C. See Unexamined Japanese Patent Publication No. Hei. 2-229476, etc. Any main group III nitride compound semiconductor light-emitting device (LED, etc.) currently put into practical use uses such a low-temperature grown buffer layer.
Incidentally, the growth temperature means the temperature of a substrate heated in execution of an MOCVD method.
The film-forming temperature of a semiconductor layer formed on a buffer layer is, however, generally about 1000° C. Hence, when the aforementioned low-temperature grown buffer layer is used, it is necessary that the substrate heated to about 1000° C. is once cooled to about 400° C. for the sake of surface cleaning and then heated to about 1000° C. again. When the substrate temperature condition is changed in a sequence of high temperature, low temperature, high temperature in the aforementioned manner, it requires much time and labor to adjust the substrate temperature itself. Hence, the large change of the substrate temperature condition was a barrier against the improvement of efficiency in production of a semiconductor device in which a GaN compound semiconductor layer is grown through such a low-temperature grown buffer layer.
In order to solve the problem, a technique to form a buffer layer at a high temperature has been disclosed in Unexamined Japanese Patent Publications Nos. Hei. 9-148626, 7-321374, 9-64477, and Sho. 59-57997, etc.
Further, with respect to nitriding of a surface of a sapphire substrate, see Unexamined Japanese Patent Publication No. Hei. 5-41541.
By the way, a general device structure of a group III nitride compound semiconductor device used for a light-emitting diode, or the like, is formed as follows. A thin buffer layer of AlN or GaN is formed on a sapphire substrate at a low temperature, and a group III nitride compound semiconductor layer, such as a GaN layer, constituting a device function is laminated on the buffer layer. In such a device, the buffer layer is guessed to be amorphous or polycrystalline. It is conceived that the buffer layer serves as a one-directionally oriented seed crystal at a high growth temperature (about 1000° C.) for the formation of a device function layer and relaxes thermal distortion based on the difference in thermal expansivity between the device function layer and the sapphire substrate.
In opposition to such an amorphous or polycrystalline buffer layer, Unexamined Japanese Patent Publication No. Hei. 9-64477 has made a proposal that a single crystalline AlN buffer layer is formed on a sapphire substrate.
Unexamined Japanese Patent Publication No. Hei. 9-64477 has asserted that a single crystalline AlN layer good in crystallinity can be grown on the sapphire substrate when a condition is satisfied so that the buffer layer is grown up to a thickness of from 20 to 300 nm at a high temperature not lower than 1300° C. and that the half-value width of an X-ray rocking curve of the buffer layer is set to be not longer than 90 sec.
AlN as a single crystal with good crystallinity can be surely grown on the sapphire substrate by the background-art method described in Unexamined Japanese Patent Publication No. Hei. 9-64477. If the crystallinity of the buffer layer is good, a device function layer of group III nitride compound semiconductors can be grown with good crystallinity on the buffer layer.
According to the present inventors' examination, however, the crystallinity of the buffer layer formed by the method described in Unexamined Japanese Patent Publication No. Hei. 9-64477 is insufficient to form such a device.